1. Technical Field
The present invention relates to a fiber optic rotation sensor and more particularly to a fiber optic rotation sensor which can be made inexpensively.
2. Background Art
Fiber optic rotation sensor, or so-called optical fiber gyroscopes (OFG) are expected to be used as the next generation rotation sensors. A variety of systems are known which employ fiber optic rotation sensors, as shown in FIGS. 6(a) to 6(f) of the accompanying drawings.
In FIGS. 6(a) to 6(f), LS denotes a light source, P a polarizer, DC a directional coupler, M a phase modulator, AOM a frequency shifter (acousto-optic modulator), L an optical fiber loop and DET a detector.
A first, phase modulation-type system is shown in FIG. 6(a) in which the timing for phase modulation of the light propagated clockwise through the fibber loop is deviated, by the propagation delay time, from that of the light propagated counterclockwise through the loop. The deviation is utilized to provide equivalently a phase difference of .pi./2. The merits of such a system are good resolution and zero-point stability as well as simplicity in its optics system. However, due to measurement of analog quantities, there are problems as to dynamic range and scale factor stability.
A second, serrodyne-type system is shown in FIG. 6(b). The cycle frequency of a sawtooth wave is controlled, and the phase difference due to the Sagnac effect is electrically corrected. This system possesses good resolution, zero-point stability, dynamic range and scale factor stability, which makes it suitable for digital processing. A disadvantage is that a high-speed optical phase modulator and strict control of the phase modulation conditions thereof are required.
A third, ring resonance-type system is shown in FIG. 6(c). The Sagnac effect is detected as a difference in resonance frequency between the light propagated clockwise through the fiber loop and the light propagated counterclockwise through the loop. The system is advantageous in that a short optical fiber may be used, and it exhibits good resolution, zero-point stability, dynamic range and scale factor stability. Also, it is suitable for digital processing. However, a laser with a narrowed spectrum of about 100 KHz is required.
A fourth, frequency modulation-type system is shown in FIG. 6(d). An optical frequency shifter AOM is inserted between the light propagated clockwise through the fiber loop and the light propagated counterclockwise through the loop, and is adjusted to provide a frequency shift for canceling out the phase difference due to the Sagnac effect. This system possesses good dynamic range and scale factor stability and is suitable for digital processing. The asymmetric property of the optical frequency shifter AOM, however, produces a bias, leading to poor stability of zero point.
A fifth, heterodyne-type system is shown in FIG. 6(e). The fact that the phase of a beat frequency component of the output from the detector is equivalent to the phase difference due to the Sagnac effect is used in this system. The system exhibits wide dynamic range and is suitable for digital processing. On the negative side, the light propagated respectively clockwise and counterclockwise through the fiber loop is separated, and the sound velocity in the optical frequency shifter AOM is dependent on temperature, resulting in poor stability of zero point.
A sixth, system of a single polarization (SP) fiber cross-polarization type is shown in FIG. 6(f). Two polarizing beam splitters PBS are used, one in front of a beam splitter BS and the other at the back of the BS, and the light from the light source LS is extracted through cross-polarization. The light beams propagated clockwise and counterclockwise respectively through the single polarization optical fiber loop L are detected by the detectors DET through the analyzer to measure the rotational angular velocity of the device. In this case, a 90.degree. phase bias is applied by a quarter-wave plate (.lambda./4 plate) in order to optimize the detection sensitivity. This system uses simple signal processing because of the absence of modulators.
The first five systems discussed above are each based on the use of a modulator; however, each have drawbacks in terms of size and cost. In addition, the fiber-optic rotation sensors of these systems each have both an optic system and an electric system present in a sensor portion thereof, causing problems when the rotation sensor is applied to telemetering or to sensing in those applications which are easily affected by electromagnetic induction. Moreover, these systems each require a lock-in amplifier (synchronous detection circuit) as a signal-processing circuit for the fiber-optic rotation sensor, Therefore, the rotation sensors based on these systems are generally large in size and are expensive.
The SP fiber cross-polarization system mentioned above, on the other hand, does not use a phase modulator but extracts the light beams propagated respectively clockwise and counterclockwise through the fiber loop by cross-polarization and does not require any lock-in amplifier in the signal detection portion thereof. Therefore, the SP system permits the use of a simpler processing circuit, as compared with the above-mentioned phase modulation systems.
The SP system does, however, possess drawbacks on a structural basis. First the polarizing beam splitter PBS is generally several millimeters square in size, due to the structure thereof. Thus, when the beam splitter is used in a fiber optic measuring system, it is necessary to collimate the light beam emitted from the optical fiber, to lead the collimated beam to the polarizing beam splitter and again to couple the split outputs from the beam splitter to the optical fiber through a lens. This requirement renders the optical fiber coupling portion expensive and large in size. Second, the SP system includes a spatial propagation portion in the coupling at the ends of the optical fiber loop. This structure causes the optical path length to vary with temperature and causes a zero-point offset problem. Therefore, it is impossible to construct the optical measuring system entirely as a single fiber-optic system.